Alumina sintered body, abrasive grains, and grindstone

ABSTRACT

Provided are an alumina sintered compact containing a titanium compound and an iron compound, wherein FeTiAlO 5  grains exist in the grain boundary of the alumina grains and the mean grain size of the FeTiAlO 5  grains is from 3.4 to 7.0 μm; and an abrasive grain and a grain stone using the alumina sintered compact.

TECHNICAL FIELD

The present invention relates to an alumina sintered compact, anabrasive grain using the alumina sintered compact, and a grind stoneusing the abrasive grain.

BACKGROUND ART

An alumina sintered compact is used in various industrial fields, makingfull use of the excellent characteristics thereof of high hardness, highstrength, high heat resistance, high abrasion resistance, high chemicalresistance, etc. In particular, it is used as a starting material(abrasive grain) of heavy grinding stones in steel industry.

Special alloys are being much used as a material for parts constitutingtransportation equipment centered on automobiles or industrialmachinery. These special alloys are hard as compared with ordinarySUS304 and others, and heavy grinding stones heretofore unknown andhaving a high “grinding ratio” are desired by the market. In this,“grinding ratio” is an index of indicating the performance of grindstones and is represented by the following formula:

Grinding Ratio=ground amount of work material(ground amount)/abrasionloss of grind stone

In general, it is considered that a grind stone requiring a smalleramount thereof to grind a larger amount of a work material could havebetter performance; however, the grinding ratio of a grind stone isinfluenced by the “hardness” and the “fracture toughness” of theabrasive grains used for the grind stone. It is considered that therewould be the following relationships between “the grinding ratio and thehardness”, and “the grinding ratio and the fracture toughness”.

(1) When the hardness of an abrasive grain is high, then the groundamount increases and therefore the grinding ratio becomes high.(2) When the fracture toughness is high, then the abrasion loss of theabrasive grain reduces and therefore the grinding ratio becomes high.

In consideration of the above (1) and (2), the numerator part in theformula of grinding ratio is influenced by the ground amount, and thedenominator part is influenced by the abrasion loss. For increasing thegrinding ratio of a grind stone, it is ideal that both the hardness andthe fracture toughness thereof are high.

As already-existing abrasive grains for heavy grinding stones, there areknown abrasive grains prepared by sintering a fine powder aluminamaterial (for example, see Patent References 1 to 3), molten aluminazirconia abrasive grains (for example, see Patent Reference 4), abrasivegrains prepared by adding a crystal grain growth inhibitor such asmagnesium oxide or the like to a high-purity fine alumina powder (forexample, see Patent Reference 5), etc.

Also proposed is a sintered material comprising aluminium material asthe main ingredient and TiO₂ added thereto (for example, see PatentReference 6). Further, as an alumina sintered compact having highhardness and high fracture toughness and excellent in abrasionresistance, proposed is an alumina sintered compact in which a solublemetal compound of Ti, Mg, Fe or the like is added to the alumina crystal(for example, see Patent Reference 7).

CITATION LIST Patent References

-   [Patent Reference 1] JP-B 39-4398-   [Patent Reference 2] JP-B 39-27612-   [Patent Reference 3] JP-B 39-27614-   [Patent Reference 4] JP-B 39-16592-   [Patent Reference 5] JP-B 52-14993-   [Patent Reference 6] JP-A 3-97661-   [Patent Reference 7] JP-A 11-157962

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

However, the abrasive grains in Patent References 1 to 5 all have highhardness but low fracture toughness or have low hardness but highfracture toughness, and these references do not concretely discloseabrasive grains that have high hardness and high fracture toughness. InPatent Reference 6, the hardness of the sintered material is evaluated,but nothing relating to the fracture toughness thereof is taken intoconsideration therein. Patent Reference 7 discloses, as the aluminasintered compact therein, only a combination of Ti and Mg and acombination of Fe and Mg, but does not concretely disclose any othercombination.

Given the situation as above, the present invention has been made andits object is to provide an alumina sintered compact capable of givingabrasive grains having high hardness and excellent in fracturetoughness, an abrasive grain using the alumina sintered compact, and agrind stone using the abrasive grain.

Means for Solving the Problems

The present inventors have assiduously studied for the purpose ofattaining the above-mentioned object and, as a result, have specificallynoted, as the compound to be contained in the alumina sintered compact,a titanium compound (especially titanium oxide) and an iron compound(especially iron oxide), and have found that the FeAlTiO₅ grains to beformed of these and alumina exhibit an excellent effect for enhancingthe fracture toughness of the alumina sintered compact. The presentinvention has been completed on the basis of these findings.

Specifically, the present invention is as described below.

[1] An alumina sintered compact containing a titanium compound and aniron compound, wherein FeAlTiO₅ grains exist in the grain boundary ofthe alumina grains and the mean grain size of the FeAlTiO₅ grains isfrom 3.4 to 7.0 μm.[2] The alumina sintered compact of the above [1], wherein the totalamount of the TiO₂-equivalent content of the titanium compound and theFe₂O₃-equivalent content of the iron compound is from 5 to 13% by mass.[3] An abrasive grain comprising the alumina sintered compact of theabove [1] or [2].[4] A grind stone having a layer of the abrasive grains of the above [3]as the working face thereof.

Advantage of the Invention

According to the present invention, there are provided an aluminasintered compact capable of giving abrasive grains having high hardnessand excellent in fracture toughness, an abrasive grain using the aluminasintered compact, and a grind stone using the abrasive grain.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 This is an action explanatory view of explaining the mode ofimpact propagation to the alumina sintered compact of the presentinvention.

FIG. 2 This shows SEM pictures of the alumina sintered compact of thepresent invention before and after impact test thereof; (A) shows thecondition of the crystalline structure before impact test (processed bythermal etching); (B) shows the condition of crack propagation afterimpact test (not processed by thermal etching).

FIG. 3 This is an action explanatory view of explaining the mode ofimpact propagation to a sintered compact of alumina alone.

FIG. 4 This shows SEM pictures of the sintered compact of alumina alonebefore and after impact test thereof; (A) shows the condition of thecrystalline structure before impact test (processed by thermal etching);(B) shows the condition of crack propagation after impact test (notprocessed by thermal etching).

FIG. 5 This is an X-ray diffraction pattern showing the result ofcompositional analysis (X-ray diffractiometry) of the alumina sinteredcompact of Example 7.

MODE FOR CARRYING OUT THE INVENTION [Alumina Sintered Compact]

In the alumina sintered compact of the present invention, there exists acrystal phase of a composite metal oxide containing Ti, Fe and Al,concretely FeAlTiO₅ grains in the grain boundary of the main crystalphase composed of a corundum crystal. The existence of the FeTiAlO₅grains provides the alumina sintered compact that gives abrasive grainshaving high hardness and excellent in fracture toughness. In particular,owing to the effect of the FeTiAlO₅ grains having higher fracturetoughness than a corundum phase, there can be obtained an aluminasintered compact having high hardness and excellent fracture toughness.The mean grain size of the FeAlTiO₅ grains can be confirmed according tothe method described in Examples to be given below.

The mean grain size of the crystal phase of the composite metal oxidewith Ti, Fe and Al (FeTiAlO₅ grains) is indispensably from 3.4 to 7.0μm, but preferably from 3.7 to 6.5 μm, from the viewpoint of increasingthe fracture toughness.

Falling outside the range of from 3.4 to 7.0 μm, the grains could not beeffective for preventing the growth of the cracks formed by fracture. Onthe contrary, falling within the range of from 3.4 to 7.0 μm, the crackdeviation effect of the FeTiAlO₅ grains can be secured well.

Here the mechanism that could provide the above-mentioned effect isdescribed below.

First, in a sintered compact of alumina alone, the crack propagationruns in the direction of the arrow Y along the grain boundary of thealumina grains 12, as shown in FIG. 3. With that, depending on theimpact level, a linear crack may form along the grain boundary, as shownin the SEM picture of FIG. 4(B). FIG. 4 shows SEM pictures of thesintered compact of Comparative Example 1 to be mentioned below; andFIG. 4(A) shows the condition of the crystalline structure before givenimpact, and FIG. 4(B) shows the condition of crack propagation aftergiven impact.

On the other hand, incorporation of a titanium compound and an ironcompound provides a crystal phase of a composite metal oxide having ahigh fracture toughness value (FeTiAlO₅ grains 10) in the grain boundaryof the alumina grains 12, as shown in FIG. 1. The FeTiAlO₅ grains 10thus exist in the grain boundary of the alumina grains 12, andtherefore, even though the crack formed by impact application growsfurther, the crack could be deviated so as to go by a roundabout routein the direction of the arrow X from the starting point of the grain 10,and consequently, the impact force is not in one direction but diffusesand is thereby relaxed. Accordingly, it is considered that the fracturetoughness value would be high as a whole.

This is known from the SEM pictures of FIG. 2 indicating the results ofimpact test. Specifically, when impact is given in the condition wherethe FeTiAlO₅ grains exist in the grain boundary of the alumina grains,as in the SEM picture of FIG. 2 (A), then the crack starting from theFeTiAlO₅ grains may go around the grains, as in FIG. 2(B).

FIG. 2 shows SEM pictures of the sintered compact of Example 3 to bementioned below, in which the gray part (tinted part) positioned in thetriple point existing in the grain boundary of the alumina grainscorresponds to the FeTiAlO₅ grain.

The alumina sintered compact of the present invention contains atitanium compound and an iron compound, wherein the total amount of thethree ingredients of the TiO₂-equivalent content of the titaniumcompound (hereinafter this may be referred to as “TiO₂-equivalentcontent”), the Fe₂O₃-equivalent content of the iron compound(hereinafter this may be referred to as “Fe₂O₃-equivalent content”) andthe alumina content is preferably at least 98% by mass.

Also preferably, the total amount of the two ingredients of theTiO₂-equivalent content and the Fe₂O₃-equivalent content is from 5 to13% by mass, more preferably from 7 to 10% by mass.

In the alumina sintered compact of the present invention, the ratio bymass of the TiO₂-equivalent content to the Fe₂O₃-equivalent content(TiO₂:Fe₂O₃) is preferably from 0.85:1.15 to 1.15:0.85, more preferablyfrom 0.90:1.10 to 1.10:0.90, even more preferably from 0.95:1.05 to1.05:0.95, in order that the sintered compact can have high hardness andhigh fracture toughness.

Regarding the relationship between the total amount of the twoingredients of the TiO₂-equivalent content and the Fe₂O₃-equivalentcontent, and the hardness, when the total amount is larger, then thehardness is lower; however, in case where the total amount of the twoingredients falls within the range defined in the present invention,then the mean Vickers hardness that is an index of hardness is, forexample, at least 16 GPa, therefore indicating the presence of apractically excellent hardness.

On the other hand, the relationship between the total amount of the twoingredients and the fracture toughness is not like the relationship tothe hardness as above; however, the present inventors have found that,within a specific range of the total amount of the two ingredients, thefracture toughness is extremely high. Specifically, when the totalamount of the two ingredients falls within the range defined in thepresent invention, then the fracture toughness value may be, forexample, at least 3.0 MPa·m^(1/2).

As described in the production method for an alumina sintered compact tobe mentioned below, use of ilmenite (titanic iron: FeTiO₃) as thestarting material containing Ti and Fe is preferred from the viewpointof the production cost.

Preferably, the alumina sintered compact of the present inventioncontains a silicon compound and/or a calcium compound that are othermetal compounds than TiO₂, Fe₂O₃ and Al₂O₃, in order that the sinteredcompact could have higher fracture toughness.

Preferably, the total amount of the SiO₂-equivalent content of thesilicon compound (hereinafter this may be referred to as“SiO₂-equivalent content”) and the CaO-equivalent content of the calciumcompound (hereinafter this may be referred to as “CaO-equivalentcontent”) is at most 2% by mass, more preferably from 0.5 to 2% by mass.

The silicon compound and the calcium compound act as a grain growingagent, and it is considered that the presence of at most 2% by mass, astheir oxides, of these compounds would unhomogenize the shape and thesize of the alumina corundum grains therefore causing deviation ofcracks. Specifically, it is considered that, owing to the existence of aspecific amount of a titanium compound and an iron compound and aspecific amount of a silicon compound and a calcium compound, therespective effects could be combined therefore efficiently providingdeviation of cracks and attaining the effect of further increasing thefracture toughness.

Here the alumina content, the TiO₂-equivalent content, theFe₂O₃-equivalent content, the SiO₂-equivalent content, theCaO-equivalent content and the metal oxide-equivalent content of othermetal compounds are determined according to a fluorescent X-rayelementary analysis method. Concretely, they are determined as follows.

First, for the measurement, a standard oxide sample of which theelementary composition is known is analyzed in wet. With the thus-found,wet analysis data taken as the standard values, calibration curvesnecessary for measurement are formed. The quantitative analysis of thesamples is carried out on the basis of the thus-formed calibrationcurves. As the measurement apparatus, usable is Panalytical's “PW2400Model”. For the measurement, preferably, the condition is such that thetube is a rhodium tube is used and the characteristic X ray is a Kα ray.Preferably, in the measurement, the tube voltage and the tube currentare varied for the individual elements. Examples of the conditions ofthe tube voltage and the tube current are shown in Table 1 below.

In this specification, the entire amount to be the denominator indetermining the individual metal oxide-equivalent content is the totalamount of all the metal elements, as their oxides, contained in thealumina sintered compact.

TABLE 1 Tube Voltage and Tube Current for Each Metal Oxide Element TubeVoltage [kV] Tube Current [mA] Al 24 120 Fe 60 48 Ti 40 72 Si 24 120 Ca40 72

[Method for Producing Alumina Sintered Compact]

Next described is a method for producing the above-mentioned aluminasintered compact of the present invention.

(Starting Material)

In the method for producing the alumina sintered compact of the presentinvention, an alumina, a titanium compound and an iron compound are usedas the starting materials. If desired, a silicon compound and/or acalcium compound may be further used. These may be in the form of acomposite oxide containing two or more of them.

Regarding the form of the starting material, there are mentioned apowder, a metal powder, a slurry, an aqueous solution, etc. In thepresent invention, preferably, the starting materials are in the form ofpowder from the viewpoint of easiness in handling them in operation. Incase where powdery starting materials are used, the cumulative mass 50%diameter (dso) of the alumina powder, the titanium compound powder, theiron compound powder, the silicon compound powder and the calciumcompound powder is preferably at most 3 μm each, more preferably at most1 μm for obtaining a homogenous mixed powder.

Here, the cumulative mass 50% diameter (d₅₀) of the powders can bedetermined according to a laser diffraction method.

The alumina powder is the starting material for forming the main crystalphase of a corundum crystal in the alumina sintered compact to beobtained, and is therefore preferably a high-purity one, and forexample, preferred is use of alumina or the like formed according to aBayer process.

The titanium compound powder and the iron compound powder may be ahigh-purity TiO₂ powder and a high-purity Fe₂O₃ powder, respectively, ormay also be in the form of a composite oxide of all or two of titanium,iron and alumina. The composite oxide includes ilmenite (titanic iron:FeTiO₃) powder, aluminium titanate powder, FeTiAlO₅ powder, etc. Theilmenite powder is more inexpensive than high-purity TiO₂ powder andhigh-purity Fe₂O₃ powder, and therefore can lower the production cost ofabrasive grains. Accordingly, use of ilmenite powder is preferred.

Here ilmenite is also called titanic iron, and a naturally-occurringiron and titanium oxide mineral, and its composition is expressed asFeTiO₃. The locality includes Australia, Norway, Russian Ural region,India, Canada, America, Malaysia, etc., and the chemical compositionvaries depending on the locality. There exist derivatives of FeTiO₃ inwhich Fe²⁺ is partly substituted with Mg²⁺.

The chemical composition of the alumina ingredient of the ingredientsconstituting ilmenite (from Queensland in Australia), and theoxide-equivalent content of iron compound, titanium compound, siliconcompound and calcium compound are shown in Table 2 below.

TABLE 2 Content of Alumina and Oxide-Equivalent Ingredients in Ilmenite(% by mass) Al₂O₃ Fe₂O₃ TiO₂ SiO₂ CaO 0.43 46.93 49.10 0.35 0.02

In case where an ilmenite powder is used, the blend ratio by mass of theilmenite powder to the alumina powder (ilmenite powder:alumina powder)is preferably from 0.05:0.95 to 0.16:0.84, more preferably from0.08:0.92 to 0.12:0.88. When the blend ratio by mass is from 0.05:0.95to 0.16:0.84, then the total amount of the two ingredients of theTiO₂-equivalent content and the Fe₂O₃-equivalent could be from 5 to 13%by mass.

In case where a silicon compound and a calcium compound are used, theSiO₂-equivalent content of the silicon compound and the CaO-equivalentcontent of the calcium compound are so controlled as to be at most 2% bymass in total, preferably from 0.5 to 2% by mass. Using these canfurther increase the fracture toughness value.

The silicon compound powder and the calcium compound powder may be ahigh-purity SiO₂ powder and a high-purity CaO powder, calcium carbonatepowder or the like, respectively; or may also be in the form of acomposite oxide of all or two of silica, calcium oxide and alumina. Asthe composite oxide, there are mentioned powders of mullite, zeolite,bentonite, gehlenite, anorthite, etc.

(Preparation of Mixture)

In the method for producing the alumina sintered compact of the presentinvention, the method of preparing the starting material mixture is notspecifically defined. For example, the following method is preferablyemployed here.

First, an alumina powder prepared according to a Bayer process and anilmenite powder (or TiO₂ powder and Fe₂O₃ powder) each in apredetermined amount are added to an aqueous medium containing polyvinylalcohol. Subsequently, for example, using an ultrasonic disperser, amedia-assisted disperser such as a planetary ball mill, a ball mill, asand mill or the like, or a medialess disperser such as Altimizer (tradename), Nanomizer (trade name) or the like, a homogeneous slurry isobtained. Next, the slurry is dried and then ground to prepare a mixture(powder) having a cumulative mass 50% diameter (d₅₀) of at most 3 μm,preferably at most 1 μm.

(Sintering of Mixture)

A shaped compact of the starting material mixture prepared in the manneras above is sintered to give the alumina sintered compact of the presentinvention having a relative density of at least 95%, preferably at least97%. Having a relative density of at least 95%, reduction in thehardness and the fracture toughness of the sintered compact to be causedby the pores and voids in the sintered compact can be prevented. Therelative density can be computed by dividing the bulk density of thesintered compact, as measured according to an Archimedian method, by thetrue density thereof.

In sintering, the mixture is shaped to have a desired form according toa known shaping method of, for example, mold pressing, cold isostaticpressing, cast molding, injection molding, extrusion molding or thelike, and thereafter the shaped compact may be sintered according to aknown sintering method, for example, according to various sinteringmethods of a hot-pressing method, a normal pressure firing method, avapor pressure firing method, a microwave-heating firing method or thelike.

Thus obtained, the alumina sintered compact of the present invention hashigh hardness and excellent fracture toughness, and is favorable for,for example, grinding, cutting, polishing or the like tools for grindingmaterials, cutting materials, polishing materials, etc., and further forabrasive grains of heavy grinding stones in steel industry.

[Abrasive Grain]

The abrasive grain of the present invention comprises the aluminasintered compact of the present invention. The alumina sintered compactof the present invention can be obtained through grinding treatment,kneading treatment, shaping treatment, drying treatment and sinteringtreatment to be attained sequentially.

[Grind Stone]

The grind stone of the present invention has a layer of the abrasivegrains of the present invention, as the working face thereof.

As the method of fixing the abrasive grains to the working face of thegrind stone of the present invention, there may be mentioned resinbonding, vitrified bonding, metal bonding, electrodeposition, etc.

As the material of the core, there may be mentioned steel, stainlessalloys, aluminium alloys, etc.

Resin bonding provides sharp cutting, but is poor in durability.Vitrified bonding provides sharp cutting and is good in abrasionresistance, but gives internal stress to the abrasive grains, wherebythe abrasive grains may be often broken or cracked. Electrodepositiongives broad latitude in shape and provides sharp cutting.

In view of the above, the fixation method may be selected in accordancewith the use of the grind stone.

Concretely, for example, in a case of a resin-bonded grind stone, theremay be employed a method comprising mixing a powder of a binder such asa phenolic resin, a polyimide resin or the like and abrasive grains, orcoating abrasive grains with a binder, then filling them in a moldfollowed by shaping it by pressing, or a method comprising mixing aliquid binder such as an epoxy resin, an unsaturated polyester resin orthe like and abrasive grains, then casting them into a mold followed bycuring it, whereby there is obtained a grind stone of the presentinvention that has a layer of abrasive grains fixed on the working facethereof.

Not specifically defined, the shape of the grind stone of the presentinvention may be suitably selected from a straight type, a cup type orthe like in accordance with the use of the grind stone.

EXAMPLES

Next, the present invention is described in more detail with referenceto Examples; however, the present invention is not whatsoever limited bythese Examples.

The properties in Examples were determined according to the methodsmentioned below.

(1) Measurement of Cumulative Mass 50% Diameter (d₅₀) of StartingMaterial Powder:

The cumulative mass 50% diameter (d₅₀) of the starting material powderwas measured according to a laser diffraction method (with Nikkiso'sMicrotrack HRA).

(2) Measurement of Mean Vickers Hardness of Alumina Sintered Compact:

As the apparatus, used was Akashi's Model “MVK-VL, Hardness Tester”.Regarding the measurement condition, the load was 0.98 N and theindenter application time was 10 seconds. Under the condition, eachsample was analyzed on 15 points, and the found data were averaged togive the mean Vickers hardness of the sample. Those having a meanVickers hardness of at least 16 GPa are free from problem in practicaluse.

(3) Mean Fracture Toughness Value of Alumina Sintered Compact:

As the apparatus, used was Matsuzawa Seiki's Model “DVK-1”. Regardingthe measurement condition, the maximum load was 9.81 N, the indenterapplication speed was 50 μm/sec, and the indenter application time was15 seconds. Under the condition, each sample was analyzed on 15 points,and the found data were averaged to give the mean fracture toughnessvalue of the sample. The computational formula is given below. Thosehaving a mean fracture toughness value of at least 3.0 MPa·m^(1/2) arefree from problem in practical use.

K _(IC)=0.026×E ^(1/2) ×P ^(1/2) ×a/c ^(3/2)

K_(IC): fracture toughness value (MPa·m^(1/2))E: Young's modulus (Pa)P: maximum load (N)a: indentation size (m)c: crack size (m)

In the present invention, the Young's modulus E is the value of alumina(3.9×10¹¹ Pa).

(4) Measurement of Mean Grain Size in Each Crystal Phase of AluminaSintered Compact:

As the apparatus, used was JEOL's Model “JSM-6510V” with which SEMpictures were taken. On the SEM pictures, the mean grain size of eachcrystal phase was measured. The mean grain size was measured as follows:According to a diameter method, the maximum length in the same directionof each grain (50 grains) was measured, and the found data were averagedto give the mean grain size of the sample.

(5) Compositional Analysis of Metal Oxide Crystal Phase Containing Ti,Fe and al of Alumina Sintered Compact:

As the apparatus, used was Panalytical's Model “X'pert PRO”. The metaloxide crystal phase was analyzed for the composition thereof under thecondition that a CuKα ray was used as the characteristic X ray, the tubevoltage was 40 kV and the tube current was 40 mA.

(6) Relative Density:

The relative density was computed by dividing the bulk density of thesintered compact, as measured according to an Archimedian method, by thetrue density thereof.

In this, it was presumed that all the iron compound and the titaniumcompound added could react to give FeTiAlO₅. With that, the true densityof alumina was considered as 3.98 and the true density of FeTiAlO₅ was4.28, and based on the proportion of the formed FeTiAlO₅ and theproportion of the remaining alumina, the true density of the sample wascomputed.

As described above, the form of the starting materials for the sinteredcompact includes powder, metal power, slurry, aqueous solution, etc. Inthis Example, from the viewpoint of easy handlability thereof inoperation, it was considered that powdery starting materials would bepreferred, and therefore powdery starting materials were used. Thechemical composition of the alumina power, the ilmenite powder, thesilicon oxide (silica) powder, the calcium carbonate powder, the ironoxide powder and the titanium oxide powder (as alumina content,TiO₂-equivalent content, Fe₂O₃-equivalent content, SiO₂-equivalentcontent, CaO-equivalent content) are shown in Tables 3 to 8 below.

TABLE 3 Chemical Composition of Alumina and Oxide-Equivalent Ingredientsin Alumina Powder (% by mass) Al₂O₃ Fe₂O₃ TiO₂ SiO₂ CaO 99.36 0.02 <0.010.01 <0.01

The above alumina powder is Showa Denko's “AL-160SG-3” and thecumulative mass 50% diameter (d₅₀) thereof is 0.6 μm.

TABLE 4 Chemical Composition of Alumina and Oxide-Equivalent Ingredientsin ilmenite (FeTiO₃) Powder (% by mass) Al₂O₃ Fe₂O₃ TiO₂ SiO₂ CaO 0.4346.93 49.10 0.35 0.02

The above ilmenite powder was from Australia, and was a product by CRL(Consolidated Rutile Limited) in Australia. Before use herein, thepowder was ground to have a cumulative mass 50% diameter (d₅₀) of 0.75μm.

TABLE 5 Chemical Composition of Alumina and Oxide-Equivalent Ingredientsin Silicon Oxide Powder (% by mass) Al₂O₃ Fe₂O₃ TiO₂ SiO₂ CaO 0 0 099.91 0

The above silicon oxide powder is Nippon Aerosil's AEROSIL 200, and itscumulative mass 50% diameter (d₅₀) is about 12 μm.

TABLE 6 Chemical Composition of Alumina and Oxide-Equivalent Ingredientsin Calcium Carbonate Powder (% by mass) Al₂O₃ Fe₂O₃ TiO₂ SiO₂ CaO 0 0 00 99.5

The above calcium carbonate powder is Wako Pure Chemicals' calciumcarbonate (special grade chemical), and its cumulative mass 50% diameter(d₅₀) is 2.3 μm. In incorporation, the amount to be added was convertedin terms of the CaO-equivalent amount thereof.

TABLE 7 Chemical Composition of Alumina and Oxide-Equivalent Ingredientsin Iron Oxide (Fe₂O₃) Powder (% by mass) Al₂O₃ Fe₂O₃ TiO₂ SiO₂ CaO 0.01996.66 0.017 2.38 0.006

The above iron oxide powder is Tone Sangyo's red iron oxide, SR-570, andits cumulative mass 50% diameter (d₅₀) is 0.5 μm.

TABLE 8 Chemical Composition of Alumina and Oxide-Equivalent Ingredientsin titanium oxide (TiO₂) Powder (% by mass) Al₂O₃ Fe₂O₃ TiO₂ SiO₂ CaO0.057 0 99.22 0 0.0011

The above titanium oxide powder is Showa Titanium's “Super Titania®GSeries”, and its cumulative mass 50% diameter (d₅₀) is 0.6 μm.

Examples 1 to 11 and Comparative Examples 1 to 7

The above alumina powder having a cumulative mass 50% diameter (d₅₀) of0.6 μm and the above ilmenite powder having a cumulative mass 50%diameter (d₅₀) of 0.75 μm were mixed in such a manner that the TiO₂content and the Fe₂O₃ content in the alumina sintered compact to beformed could be as in Table 9-1 ond 9-2, thereby preparing variousmixtures.

300 g of an aqueous solution containing 5% by mass of polyvinyl alcoholand 600 g of pure water were added to each mixture, ground and mixed ina ball mill (for 4 hours in Examples 1 to 5 and Comparative Examples 1to 6, but for 8 hours in others), thereby preparing various types ofhomogeneous slurries each having a mixture concentration of about 25% bymass.

Next, each slurry was dried at 120° C. for 24 hours, and then ground ina mortar to give a ground powder having a cumulative mass 50% diameter(d₅₀) of at most 300 μm. Each ground powder was molded in a mold under apressure of 100 MPa, and then further processed for hydrostaticpressurization under a pressure of 150 MPa to give various types ofmolded compacts.

Subsequently, each molded compact was fired in an electric furnace (airatmosphere) for 4 hours so as to have a relative density of at least95%, thereby giving various alumina sintered compacts. These were tested(evaluated) as above. The results are shown in Table 9-1 and 9-2 below.

FIG. 2 shows SEM pictures of the alumina sintered compact of Example 3before and after impact test; FIG. 4 shows SEM pictures of the aluminasintered compact of Comparative Example 1 before and after impact test.In these drawings, (A) shows the condition of the crystalline structurebefore impact test, and (B) shows the condition of crack propagationafter impact test.

Examples 12 and 13 and Comparative Examples 8 and 9

The above alumina powder having a cumulative mass 50% diameter (d₅₀) of0.6 μm, the above iron oxide powder having a cumulative mass 50%diameter (d₅₀) of 0.5 μm and the above titanium oxide powder having acumulative mass 50% diameter (d₅₀) of 0.6 μm were mixed in such a mannerthat the TiO₂ content and the Fe₂O₃ content in the alumina sinteredcompact to be formed could be as in Table 9-1 and 9-2, thereby preparingvarious mixtures.

300 g of an aqueous solution containing 5% by mass of polyvinyl alcoholand 600 g of pure water were added to each mixture, ground and mixed ina ball mill (processing time; 4 hours), thereby preparing various typesof homogeneous slurries each having a mixture concentration of about 25%by mass.

Next, each slurry was dried at 120° C. for 24 hours, and then ground ina mortar to give a ground powder having a cumulative mass 50% diameter(d₅₀) of at most 300 μm. Each ground powder was molded in a mold under apressure of 100 MPa, and then further processed for hydrostaticpressurization under a pressure of 150 MPa to give various types ofmolded compacts.

Next, each molded compact was fired in an electric furnace (airatmosphere) for 4 hours so as to have a relative density of at least95%, thereby giving various alumina sintered compacts. These were tested(evaluated) as above. The results are shown in Table 9-1 and 9-2 below.

TABLE 9-1 Physical Properties of Sintered Compact Composition ofSintered Compact Relative Mean Vickers Mean Fracture FeTiAlO₅ Presenceor (% by mass), balance Al₂O₃ Density Hardness Toughness Value meangrain Absence of Fe₂O₃ TiO₂ SiO₂ CaO (%) (GPa) (MPa · m^(1/2)) size (μm)FeTiAlO₅ Example 1 2.45 2.56 0.02 0.00 95.8 16.7 3.0 3.4 present Example2 3.41 3.57 0.03 0.00 96.7 16.5 3.4 3.7 present Example 3 3.89 4.07 0.030.00 96.7 16.4 3.9 3.9 present Example 4 4.38 4.58 0.03 0.00 96.9 16.33.6 4.1 present Example 5 4.87 5.09 0.04 0.00 97.3 16.1 3.4 4.2 presentExample 6 2.45 2.56 0.33 0.66 95.7 16.4 3.8 4.3 present Example 7 2.452.56 0.66 1.32 95.9 16.3 3.7 4.5 present Example 8 3.89 4.07 0.33 0.6695.5 16.1 4.0 5.6 present Example 9 3.89 4.07 0.66 1.32 96.6 16.0 3.85.7 present Example 10 4.87 5.09 0.33 0.66 97.0 16.0 3.8 5.8 presentExample 11 4.87 5.09 0.66 1.32 96.8 16.0 3.6 6.2 present Example 12 4.603.40 0.11 0.00 97.5 16.2 3.6 3.7 present Example 13 3.40 4.60 0.08 0.0096.8 16.1 3.5 3.8 present

TABLE 9-2 Physical Properties of Sintered Compact Composition ofSintered Compact Relative Mean Vickers Mean Fracture FeTiAlO₅ Presenceor (% by mass), balance Al₂O₃ Density Hardness Toughness Value meangrain Absence of Fe₂O₃ TiO₂ SiO₂ CaO (%) (GPa) (MPa · m^(1/2)) size (μm)FeTiAlO₅ Comparative 0.01 0.00 0.01 0.00 96.6 17.4 2.4 — absent Example1 Comparative 0.05 0.05 0.01 0.00 96.6 17.3 2.4 — absent Example 2Comparative 0.49 0.51 0.01 0.00 96.3 17.1 2.5 3.0 present Example 3Comparative 1.46 1.53 0.01 0.00 95.1 16.8 2.6 3.2 present Example 4Comparative 7.30 7.64 0.05 0.00 97.5 15.6 2.8 7.2 present Example 5Comparative 9.74 10.2 0.07 0.00 97.0 13.8 2.7 8.3 present Example 6Comparative 7.30 7.64 0.33 0.66 95.9 15.0 3.0 7.5 present Example 7Comparative 4.80 3.20 0.12 0.00 97.0 15.3 2.9 3.2 present Example 8Comparative 3.20 4.80 0.08 0.00 96.3 15.4 2.9 3.3 present Example 9

It has been confirmed through X-ray diffractiometry that, in the aluminasintered compacts of Examples 1 to 13 and Comparative Examples 3 to 9,the metal oxide crystal phase containing Ti, Fe and Al and existing inthe grain boundary of the main crystal phase composed of a corundumcrystal is a crystal phase comprising FeTiAlO₅.

FIG. 5 shows the results of X-ray diffractiometry of the aluminasintered compact of Example 7.

For the data analysis in X-ray diffractiometry, used was PANalytical'sanalysis software, “X'Pert High Score Plus”.

Using the analysis software, the crystal structure of FeTiAlO₅ wasconfirmed on the basis of the literature announced by Tiedemann et al.in 1982.

The pattern obtained through the analysis was compared with the resultof the experimental sample, and the peak was determined to be caused byFeTiAlO₅.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

-   10: FeTiAlO₅ Grains-   12: Alumina Grains-   X: Arrow Indicating the Crack Running Direction-   Y: Arrow Indicating the Crack Running Direction

1. An alumina sintered compact containing a titanium compound and aniron compound, wherein FeTiAlO₅ grains exist in the grain boundary ofthe alumina grains and the mean grain size of the FeTiAlO₅ grains isfrom 3.4 to 7.0 μm.
 2. The alumina sintered compact according to claim1, wherein the total amount of the TiO₂-equivalent content of thetitanium compound and the Fe₂O₃-equivalent content of the iron compoundis from 5 to 13% by mass.
 3. An abrasive grain comprising the aluminasintered compact of claim
 1. 4. A grind stone having a layer of theabrasive grains of claim 3 as the working face thereof.